1. Field of the Invention
The present invention is directed generally to polarization-separation device, optical scanning apparatus, and image forming apparatus. More particularly, the invention is directed to a polarization-separation device that separates light beams with different polarization directions, an optical scanning apparatus with use of the polarization-separation device, and an image forming apparatus that includes the optical scanning apparatus.
2. Description of the Related Art
Image forming apparatuses that record electrophotographic images with laser light have become widespread. Such an image forming apparatus typically includes an optical scanning apparatus that scans a surface of a photosensitive drum with laser light by using a deflector (e.g., a polygon mirror) in the axial direction of the drum being rotated to thereby form a latent image on the surface of the drum.
Upon proliferation of high-speed and color image forming in recent years, tandem image forming apparatuses that include a plurality of (typically, four) photosensitive drums have become widespread use recent years.
Tandem image forming apparatuses are likely to be upsized so as to increase the number of the drums that are used therein. Constructing units, such as an optical scanning apparatus, of an image forming apparatus compact has been desired. For compact construction, it is effective to superimpose a plurality of optical paths of scanning light beams traveling from a deflector to photosensitive drums.
For instance, disclosed is a recording apparatus that includes two laser light sources that emit two linearly-polarized laser beams, of which polarization directions are perpendicular to each other and which have undergone brightness modulation according to signals to be recorded, a polarized-light combining unit that combines the two laser light beams emitted by the laser light sources together, a deflecting unit that deflects thus-combined laser beams in the main-scanning direction, and a polarization-separation unit that separates the combined laser beams deflected by the deflecting unit so as to impinge on different spots on a scanning-recording surface in Japanese Patent Application Laid-open No. S60-32019.
Further disclosed is an optical scanning apparatus that includes a single laser light source that emits laser light, an information control unit that supplies different information pieces to two polarized-light beams obtained by polarization of the laser light from the light source, a polarization control unit that controls a degree of polarization based on information piece fed from the information control unit, a scanning unit that scans a predetermined to-be-illuminated surface with the light beam polarized by the degree determined by the polarization control unit, a separation unit that separates the light beams undergone scanning into two light beams depending on each state of polarization, and an optical-rotation control unit that controls rotation of incident laser light from the scanning unit depending on an angle of incidence on the separation unit in Japanese Patent Application Laid-open No. H7-144434.
Still further disclosed is an optical scanning apparatus that includes a light source, an optical deflector that includes a plurality of deflective reflection surfaces arranged in a sub-scanning direction, a light-beam-splitting diffractive optical element that splits a light beam from the light source into a plurality of light beams each impinging on a corresponding one of the deflective reflection surfaces, and a scanning optical system that causes the light beams deflected by the optical deflector to be converged on a surface to be scanned in Japanese Patent Application Laid-open No. 2007-279670.
However, the recording apparatus disclosed in Japanese Patent Application Laid-open No. S60-32019 is disadvantageous in that electric-field vectors of polarized light, which is expected to transmit through the polarization-separation unit, are not in some cases parallel to the incidence plane on a polarization-separating surface, causing part of the light that is expected to transmit through the polarization-separation unit to be undesirably reflected from the polarization-separation unit.
The optical scanning apparatus disclosed in Japanese Patent Application Laid-open No. H7-144434 is disadvantageous in that employment of a magnetic optical element as the optical-rotation control unit leads to higher cost. The optical scanning apparatus is also disadvantageous in that performing the optical rotation control results in increase in power consumption and amount of heat to be liberated. The optical scanning apparatus is also disadvantageous in that it is difficult to control the optical scanning apparatus so as to provide its performance stably, because the angle of optical rotation is susceptible to environmental changes such as temperature changes.